Preheat the oven to 450 degrees F, convection if possible. Slice the flank steak against the grain into thin slivers. Then toss the strips with cornstarch.
Spread the green beans out on a large rimmed baking sheet and drizzle with 3 tablespoons melted coconut oil. Sprinkle with pepper. Roast the green beans for 5 minutes.
Move the green beans to the sides of the pan, and spread the flank steak slivers across the center in a single layer. Drizzle the steak with 2 tablespoons coconut oil and roast in the oven for 8 minutes.
Meanwhile, whisk the ginger, garlic, soy sauce, brown sugar, sherry and crushed red pepper together.
Remove the sheet pan from the oven. Pour the sauce over the top of the steak. Roast again for 7 minutes. Then toss the beef and beans together and sprinkle with chopped scallions.